1. Field of the Invention
This invention relates to a method of fracturing subterranean formations surrounding oil wells, gas wells, and similar boreholes. In one aspect the invention relates to a method which employs sintered bauxite particles for maintaining a fracture in a subterranean formation in a propped condition.
2. Description of the Prior Art
Hydraulic fracturing is a well stimulation technique designed to increase the productivity of a well by creating highly conductive fractures or channels in the producing formation surrounding the well. The process normally involves two basic steps: (1) injecting a fluid at sufficient rate and pressure to rupture the formation thereby creating a crack (fracture) in the reservoir rock, and (2) thereafter placing a particulate material (propping agent) in the formation to maintain the fracture walls open by resisting forces tending to close the fracture. If stimulation is to occur, the propping agent must have sufficient mechanical strength to bear the closure stresses and provide relatively high permeability in the propped fracture.
Although a variety of particulate materials have been proposed for use as proppants, silica sand remains the only material that is used extensively. It has been long recognized, however, that sand is not the ideal proppant. Sand at closure stresses encountered in deep formations tends to disintegrate, producing fines or fragments which can drastically reduce permeability of the propped fracture.
Efforts to develop a proppant capable of bearing high stresses without excessive fragmentation have produced such material as glass beads, steel shot, and aluminum pellets. These materials, however, are quite expensive and therefore have not been widely used. The cost of proppant can best be appreciated when realizing that a typical fracturing treatment uses 30,000 pounds of proppant. High strength proppants such as glass beads currently cost from 10 to 20 times more than sand. This price can be justified only in deep wells where low priced proppants cannot bear the high stress levels. However, many of the available high strength proppants have not proven satisfactory for deep wells. Multilayers of glass beads, for example, at high stress levels under reservoir conditions do not provide high fracture permeability. Steel shot and aluminum pellets are susceptible to corrosion by formation brine.
U.S. Pat. No. 3,399,727 discloses the use of propping agent particles composed of ceramics having internal voids. These particles, preferably made by the evolution of gas bubbles within a molten ceramic (e.g. glass, porcelain or alumina), are designed to have partial crushability characteristics. According to the patent, the slow settling rate resulting from the low density particles and the partial crushability features make the proppants suitable for partial monolayer propping. However, such particles, because of their cellular structure and partial crushing characteristics, do not have sufficient compressive strength to withstand high stresses encountered in deep well fractures. Note that U.S. Pat. No. 3,399,727 states the voids reduce compressive strength and are useful at usual stress levels.
In summary, proppants for deep wells must possess sufficiently high compressive strength to avoid excessive fragmentation. They should also be inert to fluids injected into wells and reasonably priced.